1. Field of the Invention
The present invention relates to a calibration device and a calibration method for mechanical parameters of an industrial multi-jointed robot. In particular, the invention relates to a device and a method for automatically generating the position and the orientation a robot used for measuring in calibration.
2. Description of the Related Art
A method for positioning a robot with a high degree of accuracy is known, in which the relationship between inputted and outputted displacements of the robot is measured and the optimum values of the mechanical parameters of the robot is evaluated or corrected based on a measured result. Such a method for correction is generally referred to as “calibration”. As generally known, the accuracy of the calibration depends on the measurement accuracy and an error model of the mechanism. Also, the accuracy of the calibration is greatly affected by the position and the orientation of the robot used for the measurement.
There are many methods for determination and evaluation of the position and orientation of a robot used for calibration. For example, a determination method used for the measurement and an evaluated value used for the evaluation are explained in Transactions of the Japan Society of Mechanical Engineers (“C” edition), vol. 69, No. 682 (2003-6), pp. 1691-1698. Also, Japanese Unexamined Patent Publication No. 2005-201824 discloses a measurement device for determining the direction of a view line from a light receiving device mounted on the front end of a robot arm, such as a camera or a PSD, to a target to be measured, by a method easier than using conventional calibration which needs a special calibration plate. In this measurement device, a light receiving surface of the light receiving device (typically, the camera) mounted on or near the front end of the robot art receives light from the target, the robot is moved to a position such that an image of the target is captured on a predetermined point (for example, the center of a camera image), and the position of the robot is stored. Based on such a function, the relationship between the robot and the view line of the camera can be determined by moving the robot so that the position and the orientation of the light receiving device are variously changed. Once the relationship is determined, by capturing the target by using the light receiving device from different two directions, the three-dimensional position of the target may be calculated based on a principle of stereo measurement.
As described above, there are many methods for calibration. On the other hand, in the industrial field in which the robot is used, the robot position during the calibration may be determined empirically. Alternatively, the robot position may be determined by equally dividing an operation range of the robot or calculating the probability distribution equivalent to the equal division. For example, the applicant has developed a calibration method using a measurement device having a noncontact-type light receiving device such as a camera, as described in Japanese Patent Publication (Kokai) No. 2005-201824 and Japanese Patent Application No. 2006-183375.
In the calibration as described above, it is desirable to automatically determine the position and the orientation of the robot for the measurement. In other words, a fully automated calibration is desired. At this point, the position and the orientation of the robot must satisfy the following two conditions: the robot can reach the position and the orientation; and the positioning accuracy of the robot must be improved by the calibration based on the measurement result at the position and the orientation.
One major problem regarding the calibration in the industrial field is that the selected position and the orientation of the robot cannot be appropriately judged until a series of operations of the calibration have taken place, including measuring, correcting and evaluating. In particular, when the position and the orientation of the robot empirically obtained cannot be used in the industrial field and the position and the orientation are changed corresponding to the environment of the industrial field, the result of the calibration cannot be predicted until the evaluating result is obtained. Therefore, if the positioning accuracy of the robot is low, it is necessary to remeasure or measure after the position and the orientation of the robot are changed, and evaluate the positioning accuracy again.
On the other hand, in the calibration described in the Transactions of the Japan Society of Mechanical Engineers (“C” edition), vol. 69, No. 682 (2003-6), pp. 1691-1698, there are many advanced methods regarding the previous evaluation and the selection of the position and the orientation of the robot for measurement. However, there are few methods that have put into practical use so as to adapt the industrial field. Because, the position and the orientation of the robot selected offline cannot always be implemented in the industrial field. In the industrial field of the robot, the operation range of the robot is often limited due to the existence of external equipment, such as processing equipment, safety fence, column, etc. Therefore, the position and orientation of the robot must be determined in view of such a limitation.
However, it is very difficult to formulate the operation range of the robot in view of interference between the robot and the above interfering articles. Even when the formulation is successful, the number of possible combinations of the positions and the orientations of the robot is generally enormous, whereby the formulation takes a lot of time. For example, in a case where three position parameters and three orientation parameters are prepared and each parameter includes three numeric values, the number of the combinations of the position and the orientation of the robot becomes 729. Further, when three combinations should be selected among the combinations, it is necessary to check more than sixty millions patterns. In order to reduce the calculation time, in the Transactions of the Japan Society of Mechanical Engineers (“C” edition), vol. 69, No. 682 (2003-6), pp. 1691-1698, an exploratory method is used. However, the exploratory method includes a trade-off problem: if the operation range of the robot is finely divided, the exploration time is long; or if the operation range is roughly divided, the exploration result is likely to diverge from the optimum value.
Also, the measurement device used in the calibration is generally an external device, whereby an installation error necessarily exists. Depending on the magnitude of the installation error, the target is not necessarily positioned within a measurement range of the measuring device at the position and the orientation generated offline. This may be also an obstacle to the practical use of an algorithm for automatically generating the position and the orientation of the robot.